1. Field of the Invention
The present invention relates to control over allocation of country specific frequencies within an RF device, and more particularly, to an embedded digital encryption key within an RF device that permits secure software programmable frequency allocation.
2. Description of Related Art
Most industrialized countries have governmental regulations that authorize use of certain radio frequencies or radio frequency (RF) channels by particular types of radio-communication services or users under specified conditions. Such governmental regulations are considered necessary to control orderly usage of the limited band of RF frequencies, reduce interference between competing radio-communication services, and also to ensure that certain frequencies are reserved for strictly governmental purposes, e.g., police, fire department, paramedical, military, air traffic control, etc. The regulations typically require the manufacturer of an RF transmitting device to take reasonable measures to ensure compliance by the end user of the device.
Traditionally, RF device manufacturers specifically designed their products to operate only within the assigned frequencies, and not within any non-assigned frequencies. Frequency synthesis within such RF devices is easily achieved through use of specifically tuned oscillators, such as crystal oscillators, that can only operate at the tuned frequency or set of frequencies. The tuned oscillators are exceptionally frequency stable, and also are rather difficult to modify. An end user of such an RF device could not easily alter the device to operate at any non-assigned frequency. As a result, compliance by the end user with the regulations is generally assured.
This hardware-based solution to the frequency assignment issue presents a significant problem to manufacturers that market RF devices to customers located within several countries. Since the regulatory frequency assignments are not standardized among various countries, and are actually conflicting in many cases, a manufacturer must produce a different and somewhat unique version of the RF device for each country in which the device will ultimately operate. This significantly increases the manufacturing and distribution cost of the RF device. Moreover, the RF devices are considerably inflexible to changed operating conditions, such as necessitated by governmental changes in frequency assignment or user desired frequency changes to avoid other interfering RF emitters. To alter the operating frequency, the RF device generally has to be returned to the manufacturer for retuning.
To simplify the retuning process, software tunable synthesizers were developed. In a software tunable RF device, a programmable memory within the RF device stores data values that are used to control the output frequency of an oscillator, such as a voltage controlled oscillator (VCO). The programmable memory may include certain types of non-volatile storage devices, including erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), and so-called "flash" memory. In the event that it becomes necessary to retune an RF device, the manufacturer can simply alter the data values stored within the programmable memory. This retuning process can be accomplished remotely using conventional telephone lines, eliminating the need to return the device to the manufacturer. The relative ease of retuning, however, also increases the risk that an end user will unilaterally alter the stored data values and retune the RF device to a non-assigned frequency. Thus, the manufacturer cannot ensure compliance of its products with the local regulations, and as a result, may be subject to certain fines and/or penalties that result from intentional or inadvertent non-compliance by the end user.
Accordingly, a critical need exists to provide a radio-communication system having the inherent security of a hardware-based frequency synthesis with the convenience of a software programmable frequency synthesizer. Such a desirable radio-communication system would enable a manufacturer to remotely retune the RF device as necessary, while precluding unauthorized retuning by the end user of the device.